Conventional tuners are commonly categorized as the following two types: (i) a tuner that outputs quadrature detection (hereinafter referred to as I/Q detection) signals, and (ii) a tuner that outputs intermediate frequency (hereinafter referred to as IF) signals. A tuner that belongs to type (i), typified by a digital TV tuner receiving digital signals, demodulates by using an I/Q detection technique, a radio frequency (hereinafter referred to as rf) signal that has been quadrature-modulated by a base-band digital signal (hereinafter this rf signal is referred to as an rf digital signal), and then outputs an I/Q detection signal—hereinafter such a tuner is referred to as an I/Q detection tuner.
On the other hand, a tuner that belongs to type (ii) converts the frequency of the input signal into an IF, without I/Q detection, and then outputs an IF signal—hereinafter such a tuner is referred to as an IF tuner.
The latter tuner may convert not only rf signals conventionally modulated by base-band analog signals, but may also convert rf digital signals into IF signals without detection.
FIG. 4 shows a block diagram of the prior-art I/Q detection tuner. Entering input terminal 1, an rf digital signal is fed into band-pass filter (BPF) 2 and variable gain amplifier 3. After that, the signal is split into two signals to enter each branch circuit block. In one branch circuit, i.e., the upper circuit block in FIG. 4, mixer 6 receives the rf digital signal and an output signal from local oscillator 4. The output signal from oscillator 4 is provided with a 90° phase-shift by phase-shifter 5 before entering mixer 6. Phase-locked loop (PLL) circuit 15 determines the oscillating frequency generated in oscillator 4 so as to synchronize with the carrier frequency of the rf digital signal. Mixer 6 outputs a detection output signal. The detection output signal is carried, via low-pass filter (LPF) 7 and amplifier 8, to output terminal 9 as an I-signal. Similarly, in the other branch circuit—the lower circuit block in FIG. 4, mixer 10 receives the rf digital signal and an output signal from local oscillator 4, but no phase-shift is provided with the output signal. Therefore, mixer 10 outputs a detection output signal different from the output signal from mixer 6 in the upper branch. The detection output signal from mixer 10 is carried, via low-pass filter 11 and amplifier 12, to output terminal 13 as a Q-signal.
Through the process above, the I/Q detection tuner in FIG. 4 processes the two different signal components individually in each branch—one having no phase-shift and the other one having a phase-shift of 90° with respect to the phase of the rf digital signal.
Control terminal 14 works as an external controller of the amplifying level of variable gain amplifier 3. Control terminal 16 controls PLL circuit 15. Control terminal 17 works as an external controller of cut-off frequencies of low-pass filters 7 and 11.
FIG. 5 is a block diagram of the prior-art IF tuner. An rf signal fed from input terminal 21 enters first mixer 25, via BPF 22 and first AGC amplifier 23. The signal is then up-converted into a first IF signal by first mixer 25 and first local oscillator 24 to pass through first IF band-pass filter 26. First PLL circuit 33 determines the oscillating frequency of oscillator 24. The IF signal fed from BPF 26 is now down-converted into a second IF signal by second mixer 28 and second local oscillator 27. Second PLL circuit 35 determines the oscillating frequency of oscillator 27. The second IF signal enters automatic gain control (AGC) amplifying circuit—formed of variable gain amplifier 29, second IF band-pass filter 30, and variable gain amplifier 31—and goes out from output terminal 32 as an IF signal. As described above, the IF tuner shown in FIG. 5 has employed a double super heterodyne-system tuner in which an IF signal is up-converted and then down-converted to minimize image interference.
Terminals 34 and 36 control PLL circuits 33 and 35, respectively. Terminal 37 serves as an external controller of the amplifying level of variable gain amplifies 23, 29, and 31.
There is a need for a tuner receiving a digital signal to be selectively used from the two types of tuners described above as a component of a receiver according to receiver specifications, because such tuners have not been commonly used for various kinds of receivers.